The treatment of wastewater and contaminated groundwater require the removal of organic and inorganic contaminants, usually present in solid and/or dissolved form, before their discharge into the receiving waters. The organic contaminants include sources of COD/BOD such as proteins, lipids and polysaccharides as well as hazardous compounds such as aromatic and aliphatic hydrocarbons. Examples of the latter group include gasoline and diesel fuel, polycyclic aromatic hydrocarbons, phenols, chlorophenols, alkylated benzenes, tetrachloroethylene (PCE) and trichloroethylene (TCE). The nitrogenous and phosphorus compounds, which are among the most undesirable inorganic contaminants of wastewater and contaminated groundwater, also need to be removed during the treatment process.
Biological treatment processes use a variety of microorganisms such as bacteria, protozoa and metazoa for an efficient and complete biodegradation of contaminating compounds in wastewater and contaminated groundwater and landfill leachates. The removal of carbonaceous, nitrogenous and phosphorus-containing compounds is carried out by bacteria, whereas protozoa and metazoa mostly contribute to the reduction of turbidity since they graze on bacteria as a food source. The organic charge in the wastewater is often measured by chemical oxygen demand (COD) or biochemical oxygen demand (BOD). These parameters define the overall oxygen load that a wastewater will impose on the receiving water. During biological treatment processes, organic substances are removed since these substances serve as the source of carbon in the microbial metabolism. Nitrogen and phosphorus are also consumed by microorganisms as essential nutrients to support microbial growth during assimilatory processes, while excess amounts of nitrogenous compounds is removed during dissimilatory microbial nitrogen metabolism where they are transformed to molecular nitrogen and released into the atmosphere. The remaining phosphorus may be removed by the “luxury phosphorus uptake” process where special groups of microorganisms accumulate phosphorus and store it as poly-phosphorus compounds, thus removing it from the system during sludge disposal.
Nitrogen and phosphorus have been recognized as major contributors to eutrophication, a process that supports the growth of algae and other undesirable organisms in the receiving waters and diminishes the concentration of dissolved oxygen, thus threatening aquatic life. Therefore, stringent criteria have been introduced demanding the reduction of these nutrients below certain levels that are established by environmental agencies, before the effluent of treatment plants can be safely disposed to the receiving waters.
In general, the success of biological treatment systems depends on the concentration of biomass and the mean cell residence time (MCRT), as well as the ability of treatment system to separate sludge from the treated liquid. These parameters control the efficiency of treatment and the quality of effluent. Suspended-growth biological treatment systems that are based on activated sludge processes have difficulties in maintaining an adequate concentration of active biomass, and to effectively separate solids from liquid. They also produce large amounts of sludge and have a slow adaptation to fluctuating influent conditions. These problems have been addressed in the design of fixed-film treatment systems that use attached microbial biomass, immobilized on a support material. The immobilized cells grow and produce microbial biofilm, containing a consortium of microorganisms that changes with time and within the thickness of the biofilm. Fixed-film systems can operate in aerobic, anoxic or anaerobic modes depending on the nature of the contaminating compounds. These systems offer microbial diversity and prevent the washout of biomass. They also offer a higher MCRT and ease of operation relative to the separation of biomass from liquid. However, fixed-film systems have a lower rate of contaminant removal compared to suspended-growth treatment systems since the rate of removal in these systems is controlled by mass transfer and diffusion within the microbial biofilm. Aerobic fixed-film systems treating high organic concentrations have a limited capacity because of oxygen transfer limitations. Moreover, the build-up of a thick and heavy biofilm, resulting from high concentrations of organic substances may cause clogging, seriously disrupting the operation of the treatment system. Therefore, fixed-film systems have a limited application in the treatment of high organic load wastewaters. Another disadvantage of fixed-film treatment systems is that they usually operate in plug-flow mode and do not offer the homogenous environment provided by completely mixed reactors. In these systems, there is a high concentration of contaminants at the influent end of the reactor and the microorganisms will be subjected to the full concentration of contaminants that may be toxic.
All of the technologies discussed above were originally designed for secondary treatment, i.e. removal of carbonaceous compounds and solid-liquid separation and not to remove nutrients, notably nitrogenous and phosphorus compounds or halogenic substances from wastewater or contaminated groundwater and landfill leachate that require the presence of different environments with different levels of dissolved oxygen concentration and oxidation-reduction potential. These technologies also cannot stabilize the produced sludge and need supplementary vessels for this process.
In order to meet stringent discharge criteria including nitrogen and phosphorus removal, wastewater treatment plants usually upgrade their performance by using add-on technologies such as biological nutrient removal (BNR) systems. The theories of biological nitrogen and phosphorus removal mechanisms demonstrate that nitrogen removal needs the presence of aerobic and anoxic environments, while the removal of phosphorus demands the presence of anaerobic and aerobic environments in the treatment system.
Conventional wastewater treatment technologies, originally developed for the removal of carbonaceous compounds (BOD) and suspended solids, accommodate nutrient removal by providing additional aerated, anoxic or anaerobic units in series, along with various internal recycle streams to achieve the required removal of nitrogen and/or phosphorus. These modifications have increased the complexity of the treatment systems and complicated their proper design and optimization. Nitrification, the first step in the biological nitrogen removal mechanism that involves the conversion of ammonia nitrogen to nitrate nitrogen, requires an aerobic environment and is achieved in all aerobic reactors if the right operating conditions such as the liquid pH, carbonate concentration, and sludge retention time exist. Denitrification, i.e. the transformation of nitrate nitrogen to molecular nitrogen, can be accomplished by the addition of an anoxic activated sludge reactor or fixed-film system. An easily degradable carbon source must be present for the denitrification process. If the treatment system cannot supply the required carbon source, then methanol, ethanol, acetic acid or a different compound must be added. In these combined processes, the wastewater or contaminated groundwater is first fed into the anoxic denitrification reactor. The effluent from the anoxic reactor is fed into the aerobic reactor. A sufficient retention time in the aerobic reactor is needed to ensure a complete oxidation of carbonaceous compounds as well as adequate growth and proliferation of slow-growing nitrifiers to carry out the nitrification process and convert ammonia-nitrogen to nitrate-nitrogen. Sequencing batch reactor (SBR) systems have been used for biological nitrogen removal by incorporating anoxic-aerobic sequencing bioreactors. Instead of using an external carbon source for denitrification, it is quite possible to design a nitrification-denitrification system that uses the carbon present in the raw wastewater as well as the carbon released from the endogenous respiration of microbial sludge. In these systems, nitrification and denitrification occur in a singe vessel with alternating aerobic and anoxic zones. Alternatively, aerobic and anoxic zones may be present in separate vessels positioned in series. Sufficient recycle is required to prevent the effluent from containing excessive ammonia concentrations. Several systems have been developed along with these design elements. The two most successful have been the denitrifying oxidation ditch and the Bardenpho process.
In the denitrifying oxidation ditch, an anoxic zone is added inside the aerobic reactor. The influent is added to the anoxic zone and the effluent is withdrawn from the aerobic reactor. Solids are then separated from the liquid by settling in a clarifier. One of the most common design modifications for enhanced nitrogen removal is known as the Modified Ludzack-Ettinger (MLE) process. In this process, an anoxic tank is added upstream of the oxidation ditch along with mixed liquor recirculation from the aerobic zone to the tank to achieve higher levels of denitrification.
Another variation of aerobic/anoxic systems for nitrification/denitrification is the four stage Bardenpho process that has two aerobic and two anoxic vessels. Carbon from the untreated wastewater and from endogenous decay of microorganisms is used for denitrification by returning the aerobically treated wastewater to the initial anoxic zone.
In order to achieve phosphorus removal as well as nitrogen removal, the incorporation of an additional anaerobic zone in the treatment system is necessary. Two such configurations are the A2/O process and the five-stage Bardenpho process. The A2/O system includes anaerobic, anoxic and aerobic zones. In this process, the untreated wastewater is first added to the anaerobic zone where soluble phosphorus is released and VFAs are uptaken by the phosphorus accumulating microorganisms (PAOs). The effluent of the anaerobic tank is fed into the anoxic zone for the reduction of nitrate and its conversion to nitrogen. The effluent of the anoxic zone flows to the aerobic zone for BOD removal and nitrification. The separation of solids and liquid takes place in a clarifier. Two recycle streams are present in this process: one from the clarifier to the anaerobic zone to return a portion of the separated sludge, and the second one from the aerobic to the anoxic zone carrying nitrate for the denitrification process. The ability of the A2/O process to provide anaerobic dehalogenation has raised interest in this process for the treatment of groundwater and landfill leachate contaminated with hazardous chemicals such as chlorinated aliphatic compounds, recognized as common contaminants of soil and groundwater around the world.
In the Bardenpho system, there are two aerobic and two anoxic zones. Similar to the A2/O process, there are also two recycle streams between the clarifier and the anaerobic zone, and between the first aerobic and anoxic zones. In this configuration, a more complete removal of nitrogen is achieved. Moreover, the anaerobic zone will not receive nitrate in the recycle stream, thus a better phosphorus removal process can also take place. The five-stage Bardenpho process has a high nutrient removal capacity and can remove high concentrations of nitrogen and phosphorus from wastewater and contaminated groundwater.
As learned from the above description, most conventional nutrient removal systems are multi-vessel/multi-zone processes that have complicated designs and large footprints, and require high operator attention and maintenance requirements.
An alternative nutrient-removal, wastewater treatment technology that uses a multi-zone system with different environmental conditions is the Integrated Biologically Active Clarifier (IBAC). This technology that is currently in commercial operation in Quebec, Canada, combines biological treatment, solid-liquid separation and sludge stabilization in a single vessel. This treatment system has three vertically-stacked biological zones having aerobic, anoxic and anaerobic environments as well as a clarification zone. The vertical stacking of the treatment zones causes the settlement of heavy solid material including high-density biological flocs to the anaerobic zone that is located at the bottom of the reactor where anaerobic biodegradation occurs. The system does not use any recycle streams either for sludge or mixed liquor. The mixing and liquid recirculation is provided by the introduction of air into the aerobic zone. This technology suffers from a series of problems that seriously upset the operation of the treatment system. They include the periodic rise of sludge in the clarification zone due to excessive gas production in the bottom anaerobic zone, inconsistent nutrient removal, and poor settleability of solids. In addition, this technology does not permit proper control and optimization of the biological treatment and solid-liquid separation processes due to the occurrence of all different processes in a single vessel and the existing interactions among them.
Several wastewater and groundwater treatment technologies for the removal of organic carbon and nutrients are described in the patent literature. Examples of techniques dealing with the removal of carbonaceous material, nitrogen and/or phosphorus form wastewater can be found in U.S. Pat. Nos. 4,488,968; 4,948,510; 5,128,040; 5,160,043; 5,288,405; 5,518,618; 5,601,719; 5,651,891; 5,972,219; 6,139,743; 6,372,138; and 6,413,427.
U.S. Pat. No. 5,518,618 relates to a system for the treatment of nitrogenous wastewater by biological nitrification and denitrification, wherein the wastewater along with microorganisms immobilized on a carrier material flow alternately in a downward flow in an oxygen-depleted chamber and in an upward flow in an oxygen-rich chamber. The aerated reactor chamber may be partly divided into a riser and a downcomer which allow, as a result of the air supply, mass circulation to take place in this chamber. The oxygen-depleted reactor chamber contains a degassing chamber and a settling chamber at the top in order to separate the liquid from the solid sludge. The aerated chamber is supplied with a gas delivery system for producing an upward flow of waste water and a gas discharge located above it. The aerated chamber is also supplied at the top with an overflow to the oxygen-depleted reactor chamber. This treatment system does not have any anaerobic zone to promote phosphorus removal and does not stabilize sludge.
U.S. Pat. No. 6,139,743 relates to a suspended-growth treatment system that includes a multiplicity of tanks, some with several compartments, including an anaerobic/anoxic reaction tank, an aeration tank and a settling tank. The settled sludge is further returned into the anaerobic compartment. The treatment system is capable of reducing the carbon, nitrogen and phosphorus compounds in the wastewater. However, it requires that an external carbon source be provided to each compartment of the anaerobic/anoxic reaction tank to support the nitrogen and phosphorus removal processes. This practice adds to the operational cost of treatment. The treatment system has a large footprint that requires large areas for its setup and operation. Like other suspended-growth treatment technologies, there is also a high production of sludge and an associated high cost for its disposal.
U.S. Pat. Nos. 5,128,040 and 5,651,891 use a series of aerobic and anoxic/anaerobic tanks for the removal of BOD, improvement of solid settleability and reduction of nitrogen and phosphorus in wastewater. Suspended-growth processes, e.g. activated sludge, or attached-growth processes, e.g. trickling filter or rotating biological contactor may be used for BOD reduction and nitrification in these treatment systems. One of the specifications teaches the use of two fermentation tanks for the production of VFAs, required in the denitrification and phosphorus removal process, and its return to the anoxic/anaerobic tank. These inventions improve the efficiency of solids removal in trickling filter/solids contact processes. However, they have a limited phosphorus removal capacity, use a multiplicity of tanks, need a very large area for operation and have several recycle streams, considerably increasing their complexity, maintenance requirement and cost of operation.
U.S. Pat. No. 5,601,719 discusses a similar treatment system for the removal of BOD, nitrogen and phosphorus from wastewater containing a series of aerobic, anoxic and anaerobic vessels, a sludge fermenter, a secondary clarifier, and a supplemental substrate source with several recycle streams. The inclusion of an anaerobic fermentation stage in the treatment process has also been discussed in several other specifications such as U.S. Pat. Nos. 4,999,111, 5,013,441, and 4,874,519.
U.S. Pat. No. 5,480,548 relates to a step feed activated sludge process including anaerobic-anoxic-aerobic zones for biological nitrogen and phosphorus removal. The system contains multiple step feed points arranged in a series of consecutive treatment stages and a series of recycle lines carrying the effluents of anoxic and anaerobic zones as well as the return activated sludge to different zones. The system is purported to achieve reductions of phosphorous and ammonia greater than 90% and 97%, respectively, and a 50% reduction in total nitrogen.
U.S. Pat. No. 4,056,465 relates to a modified activated sludge system where BOD-containing wastewater and recycled sludge are initially mixed under anaerobic conditions and in the absence of nitrate or nitrite thereby promoting the production of the desired type of microorganisms. The effluent of this stage is then sent to an aerobic tank where BOD removal, phosphorus uptake and nitrification take place. Nitrates and nitrites are removed by interposing an anoxic treating zone between the anaerobic zone and the aerating zone.
U.S. Pat. No. 4,948,510 describes a process containing a plurality of basins which may be individually controlled to achieve anaerobic, anoxic or aerobic conditions. The basins are reconfigurable in that the flow of effluent to a basin, transfer of mix liquor between basins and effluent discharge from a basin can be varied to create a treatment cycle which has features of both continuous and batch processes while minimizing recycle rates and hydraulic level changes.
U.S. Pat. No. 6,063,273 discloses an apparatus for the biological purification of wastewater, the apparatus having a column containing an upflow anaerobic sludge bioreactor (UASB) at the bottom and an aerobic reactor at the top. The two reactors are separated from one another by a partition in which openings are provided to allow the anaerobic effluent through into the aerobic reactor. The partition forms a buffer zone preventing the biomass from the anaerobic zone to mix with the biomass from the aerobic zone.
Several other treatment technologies, mostly based on activated sludge processes; using multi-vessel or multi-zone aerobic/anoxic/anaerobic systems for the complete or partial removal of nitrogen and phosphorus from municipal or industrial effluents have been reported. Examples of such technologies are found in U.S. Pat. Nos. 4,056,465; 4,271,026; 4,488,967; 4,500,429; 4,522,722; 4,948,510; and 5,137,636.
Examples of treatment systems and apparatus for groundwater and landfill leachate contaminated with hazardous material have been discussed in U.S. Pat. Nos. 4,290,894; 4,442,005; 5,080,782; 5,389,248; 5,413,713; 5,578,202; 5,766,476; 5,804,432; 5,922,204; 6,159,365; and 6,461,509.
U.S. Pat. No. 5,922,204 discusses a treatment system containing a plurality of reactors connected in series. By means of process kinetics control, the treatment system accomplishes biological denitrification and metal precipitation in the first reactor followed by biological sulfate reduction and the production of saturated and unsaturated hydrocarbons in the second reactor, followed by methanogenesis and finally aerobic respiration in the final reactor.
U.S. Pat. No. 5,413,713 discloses a method for increasing the rate of anaerobic bioremediation in a bioreactor by recirculating to the bioreactor a given portion of a pollution stream that has flowed through a passageway containing material on whose surface anaerobic microorganisms can attach or become immobilized. The flow rate of recirculating liquid is adjusted to the level that it would slough from the surface and return to the stream at least a portion of the attached or immobilized microorganisms from the surface.
U.S. Pat. No. 5,080,782 relates to a bioremediation vessel containing a gas injection system, a nutrient addition system, and a continuously regenerating culture of microorganisms which biodegrade hazardous substances. The bioremediation vessel contains a support material for the immobilization of microorganisms while a fraction of microorganisms slough off the support medium and disperse into the treated ground water.
U.S. Pat. No. 6,461,509 discusses a treatment system that uses an aerobic packed-bed bioreactor to degrade organic material in the contaminated water. An additive consisting of at least one of a vegetable extract and a nutrient medium, is added to the contaminated water immediately before entering the packed bed to stimulate the production of exo-enzymes by the bacteria.
U.S. Pat. No. 5,578,202 relates to a water processing system for processing highly contaminated water that has a plurality of processing chambers defined by partition walls. The system comprises an anaerobic chamber, an aerobic chamber, a buffering chamber, and a recycle for recycling part of the water received in the buffering chamber back to the anaerobic chamber, and a filter material forming a buoyant filter layer in an upper part of the water received in the anaerobic chamber. The filter layer can be highly resistant against clogging, and can be easily maintained with the result that the overall system can be made both simple and economical.
U.S. Pat. No. 6,159,365 discusses an encased packaged modular type unit for treatment of contaminated water comprising a separation compartment to separate solids and, if present, oil and grease in contaminated water, a fluidized bed reactor assembly containing aeration zone(s), internal recirculation zone(s), clear effluent zone(s), and mixing/degassing zone(s). The fluidized bed contains suspended viable biomass, a physico-chemical reagent or a mixture of both biomass and physico-chemical reagent for the removal of contaminating compounds, and a compartment wherein excess sludge is thickened and removed.
Most of prior art solutions suffer from complicated designs, high maintenance requirements or large footprints as well as a limited capacity to address the treatment of groundwater or landfill leachate contaminated with a mixture of contaminating compounds of organic and inorganic nature. Examples of such contaminations include mixtures of hydrocarbons (e.g. diesel fuel, jet fuel or gasoline) with nitrate and phosphorus, commonly resulting from the combined agricultural and airport or military activities. Other examples include mixtures of aromatic hydrocarbons and halogenated hydrocarbons (e.g. PCE and TCE), and sometimes metals. These kinds of contaminations require the simultaneous presence in the treatment system of diversified groups of microorganisms as well as different environmental conditions including different levels of dissolved oxygen concentration and redox potential for their complete treatment. Provisions have to be made for adequate biomass growth and maintenance of all different microbial groups, effective solid-liquid separation, sludge stabilization, and proper optimization and control of environmental conditions in the multiple zones of the treatment system.